Logging in is done with your Google account (at least, so did we). For that, see below for 3rd party logins: you should revoke access from the 3rd party. Of course, you may want to delete the application from your device too (e.g. the Firefox plugin or Android app).
URLS:
http://www.feedly.com/
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.